The present invention relates to a method and a system for cooling a high temperature strip material in two steps.
As an example of equipment with a system for cooling a high temperature strip material, a hot dip galvanizing system is shown in FIG. 3. This system comprises a hot dip galvanizing tank 60, a heater 71, a soaking device 72, and a mist cooler 80 as a cooling device.
According to the above system, a steel strip 50 is galvanized in the hot dip galvanizing tank 60, moved vertically upward, and heated with the heater 71 to alloy the zinc with the steel. The alloyed steel strip 50 is soaked over its entire width by means of the soaking device 72. This steel strip 50 traveling in a cooling zone C is cooled with the mist cooler 80 from 520xc2x0 C. to 200xc2x0 C., and carried horizontally by a deflector roll 90.
The mist cooler 80 is composed of mist sprayers 81 disposed in opposing positions at both sides of the ascending steel strip 50. Each mist sprayer 81 comprises water supply pipes 82 and air supply pipes 83 arranged vertically in rows such that each air supply pipe 83 is mounted inside each water supply pipe 82 in a double-pipe configuration. Each water supply pipe 82 has many nozzle holes made along the width of the steel strip 50, and each air supply pipe 83 has many nozzle holes made along the width of the steel strip 50. The mist cooler 80 forms mists 86 from water 84 in the water supply pipes 82 by jetting air 85 through the nozzles of the air supply pipes 83, and directs the mists 86 toward the surfaces of the steel strip 50 to cool it.
With the foregoing mist cooler 80, mists 86 with a constant water volume density were sprayed on both sides of the steel strip 50 throughout the cooling zone C to cool the steel strip 50. At a site in the cooling zone C where the temperature of the steel strip 50 was about 350xc2x0 C. or lower (i.e., an upper portion of the cooling zone C), however, the mists 86 adhering to the surfaces of the steel strip 50 underwent transition boiling, rapidly cooling the steel strip 50. Transition boiling refers, in terms of water, to a phenomenon involving transition from a state of cooling with water vapor to a state of direct cooling with water, or to a state of cooling with a mixture of water and water vapor. This phenomenon takes place at about 350xc2x0 C. Thus, nonuniform temperature distribution of the steel strip 50 was liable to occur, thereby deforming the steel strip 50, resulting in its malformation.
The present invention has been accomplished to solve the above-described problems.
According to a first aspect of the present invention, there is provided a method for cooling a strip material, comprising:
passing the strip material, which is traveling, through a high temperature cooling zone and a low temperature cooling zone in this order, to cool the strip material with a high water volume air-water mixture in the high temperature cooling zone, and then cooling the strip material with a low water volume air-water mixture in the low temperature cooling zone.
According to a second aspect of the present invention, there is provided a method for cooling a strip material, comprising:
passing the strip material, which is traveling, through a high temperature cooling zone and a low temperature cooling zone in this order, to cool the strip material with a high water volume air-water mixture in the high temperature cooling zone to a temperature in the vicinity of a temperature at which transition boiling occurs, and then cooling the strip material with a low water volume air-water mixture in the low temperature cooling zone while suppressing transition boiling.
The air-to-water ratio of the high water volume air-water mixture may be about 1500, and the air-to-water ratio of the low water volume air-water mixture may be about 5000.
The above method may further comprise:
cooling the strip material in the high temperature cooling zone to a temperature in the vicinity of a temperature at which transition boiling occurs; and
cooling the strip material in the low temperature cooling zone to a predetermined temperature.
In the above method, the passing step may include the sub-steps of:
cooling the strip material to about 350xc2x0 C. in the high temperature cooling zone, and
cooling the strip material from about 350xc2x0 C. to a predetermined temperature in the low temperature cooling zone.
According to a third aspect of the present invention, there is provided a system for cooling a strip material, comprising:
a high temperature cooling zone and a low temperature cooling zone established as cooling zones, in which the strip material is cooled with a high water volume air-water mixture in the high temperature cooling zone, and cooled with a low water volume air-water mixture in the low temperature cooling zone.
In this system, the air-to-water ratio of the high water volume air-water mixture may be about 1500, while the air-to-water ratio of the low water volume air-water mixture may be about 5000.
In the above system, the high temperature cooling zone may cool the strip material to about 350xc2x0 C., while the low temperature cooling zone may cool the strip material from about 350xc2x0 C. to a predetermined temperature.
According to a fourth aspect of the present invention, there is provided a system for cooling a traveling strip material, comprising:
a high temperature cooling zone and a low temperature cooling zone established along a direction in which the strip material travels;
a high water volume air-water mixture cooler installed in the high temperature cooling zone for cooling the strip material with a high water volume air-water mixture to a temperature in the vicinity of a temperature at which transition boiling occurs; and
a low water volume air-water mixture cooler installed in the low temperature cooling zone for cooling the strip material with a low water volume air-water mixture while suppressing transition boiling.
The high water volume air-water mixture cooler may spray high water volume mists onto both sides of the strip material, and the low water volume air-water mixture cooler may spray low water volume mists onto both sides of the strip material.
The high water volume air-water mixture cooler may include a multiplicity of spray pipes arranged vertically, each spray pipe having a water supply pipe for supplying a high water volume, and an air supply pipe mounted inside the water supply pipe, the water supply pipe extending in the direction of the width of the strip material and having a plurality of nozzle holes drilled facing a surface of the strip material, and the air supply pipe having a plurality of nozzle holes drilled in the direction of the width of the strip material. The low water volume air-water mixture cooler, on the other hand, may include a multiplicity of spray pipes arranged vertically, each spray pipe having a water supply pipe for supplying a low water volume, and an air supply pipe mounted inside the water supply pipe, the water supply pipe extending in the direction of the width of the strip material and having a plurality of nozzle holes drilled facing a surface of the strip material, and the air supply pipe having a plurality of nozzle holes drilled in the direction of the width of the strip material.
According to a fifth aspect of the present invention, there is provided a galvanizing system for galvanizing a strip material, comprising:
a hot dip galvanizing tank which galvanizes the strip material;
a heater that heats the galvanized strip material;
a soaking device that soaks the heated strip material;
a high temperature cooling zone which cools the soaked strip material by spraying a high water volume air-water mixture thereon; and
a low temperature cooling zone which cools the soaked strip material, after cooling in the high temperature cooling zone, by spraying a low water volume air-water mixture thereon.
The hot dip galvanizing tank may contain molten zinc.
The present invention described above is carried out, for example, as a cooling system in hot dip galvanizing equipment. That is, this invention is applied in cooling a steel strip that has passed through a heater and a soaking device after undergoing hot dip galvanization. When the invention is applied as a cooling system in hot dip galvanizing equipment, the steel strip after hot dip galvanization is cooled with a high water volume air-water mixture (high water volume mists) in the high temperature cooling zone, and then cooled with a low water volume air-water mixture (low water volume mists) in the low temperature cooling zone. As a result of this two-step cooling, the temperature at which transition boiling occurs is lowered. Since the steel strip is not rapidly cooled, its temperature distribution becomes uniform. Thus, malformation of the steel strip due to thermal deformation does not occur.